Despite improved immunosuppression and routine PRA screening, alloreactive antibodies (alloAb) mediate a significant proportion of acute allograft rejection episodes and contribute to the development of chronic rejection. Production of pathogenic alloAb isotypes requires interactions between B cells and helper CD4 T cells specific for donor antigens. Typically, this help occurs in germinal centers within secondary lymphoid organs and is dependent on CD40/CD154 costimulatory pathway. However, the T cell repertoire of many humans contains alloreactive memory T cells that are resistant to immunosuppression or costimulatory blockade. Our preliminary data show that donor-reactive memory CD4 T cells induce higher titers of alloAb compared to naive CD4 T cells even in the absence of conventional germinal center formation and CD40/CD154 interaction. However, the mechanisms underlying the observed high alloAb titers as well as anatomical sites and molecular requirements of help by memory CD4 T cells are still unknown and need to be tested. The goal of this study is to identify functional phenotype of memory CD4 T cells providing help for alloreactive B cells and to investigate the characteristic features and requirements for this help. We have developed a mouse model of kidney transplantation to study donor-specific alloAb responses induced by memory CD4 T cells. We hypothesize that compared to naive cells, memory CD4 T cells initiate enhanced development of GCs and greater proliferation of B cells, promote production of alloAb with higher affinities for donor antigens and induce enhanced development of long-lived Ab secreting plasma cells and memory B cells. We further propose that while follicular helper memory T cells are superior in inducing alloAb responses compared to other lineages, help by memory CD4 T cells can occur outside of typical germinal centers and can bypass the requirement for CD40/CD154 interaction via alternative molecular pathways such as TLR and BAFF/APRIL signaling. We will test this hypothesis in three Specific Aims: Aim 1. To investigate the mechanisms of superior alloantibody production induced by donor-specific memory CD4 T cells in response to renal allografts. Aim 2. To identify functional phenotype of memory CD4 T cells capable of providing help for alloantibody production after renal allograft placement. Aim 3. To determine the location and molecular requirements of help provided by memory CD4 T cells for alloantibody production.